1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device that uses semiconductor with a crystalline structure, more specifically to a method for manufacturing a device such as a thin film transistor, a thin film diode, or a field emission device, which uses a crystalline semiconductor film.
2. Description of the Related Arts
A semiconductor circuit, an electro-optical device, or a light emitting device, which has a driving circuit and a pixel portion formed using a semiconductor device (typically, a thin film transistor, a thin film diode, or a field emission device), and an electronic device that uses the semiconductor circuit, the electro-optical device, or the light emitting device have actively been manufactured. As an active layer of the semiconductor device, a semiconductor film is used, and in particular, a crystalline silicon film is used to realize high field effect mobility.
An element of electrical characteristics of a thin film transistor depends on quality of a semiconductor film. In particular, field effect mobility depends on crystallinity of a semiconductor film, and the field effect mobility is directly concerned with response properties of a thin film transistor and display ability of a display device manufactured using the thin film transistor for a circuit.
Now, in order to form a thin film transistor with high electrical characteristics, the technique of performing one or both of thermal annealing and laser annealing with respect to an amorphous semiconductor film formed on an insulating substrate such as glass to improve crystallinity of the amorphous semiconductor film, and further to form a crystalline semiconductor film, has been widely studied.
However, when thermal annealing or laser annealing is used with respect to an amorphous semiconductor film formed with plasma CVD or sputtering to form a crystalline semiconductor film, the crystal orientation is likely to be oriented in any direction and it is difficult to control the crystal orientation. Accordingly, it is a cause of limiting electrical characteristics of a thin film transistor to use a crystalline semiconductor film crystallized by thermal annealing or laser annealing as an active layer of the thin film transistor.
As a method for crystallizing an amorphous semiconductor film, a method described in Japanese Patent Laid-Open No. Hei 07-183540 can be given. The crystallizing method will be briefly explained below. First, an amorphous semiconductor film is doped with a slight amount of metal element such as nickel, iron, cobalt, palladium, or platinum typically. For doping with the metal element, plasma treatment, evaporation, ion implantation, sputtering, or application of solution may be used. After that, when the amorphous semiconductor film is heated in a nitrogen atmosphere at a low temperature, for example, 550° C., a reaction of crystallization occurs with a compound of the metal element and an element of the semiconductor film (typically, nickel silicide, iron silicide, cobalt silicide, platinum silicide, or palladium silicide) as a nucleus so that a crystalline semiconductor film is formed.
It is confirmed that a semiconductor film can be enhanced to have the crystal orientation in a single direction and a semiconductor film composed of crystal grains with a large grain size can be formed when the method is used for crystallization. In addition, it is also confirmed that there are few defects in the crystal grains. Therefore, when a moving direction of a carrier is aligned with an extending direction of a formed crystal grain, it is possible to reduce extremely the number of times a carrier cuts across a grain boundary. Accordingly, it becomes possible to reduce fluctuation in on-current (drain current flowing in an on-state of a thin film transistor), off-current (drain current flowing in an off-state of a thin film transistor), threshold voltage, S-value, and field effect mobility, and electrical characteristics are remarkably improved. It is noted that conditions such as an appropriate heating temperature and an appropriate heating time depends on a dosage of the metal element and a state of the amorphous semiconductor film.
Since the amorphous semiconductor film is doped with the metal element, however, the metal element (a silicide of the metal element in the case of the semiconductor film of silicon, which exists typically as nickel silicide, iron silicide, cobalt silicide, platinum silicide, or palladium silicide) remains in the semiconductor film or on a surface thereof. The remaining metal element becomes a path of leakage current to increase off-current of a thin film transistor, which has trouble of becoming a cause of fluctuation in electrical characteristics among thin film transistors. Therefore, it is necessary to remove the metal element in a semiconductor film or to reduce a concentration of the metal element (hereinafter, referred to as a gettering process) after crystallizing the amorphous semiconductor film.
Consequently, the present applicant discloses a method for removing a metal element in a crystalline semiconductor film in each of Japanese Patent No. 3107941 (pages to and FIGS. 3A to 3E), Japanese Patent Laid-Open No. Hei 07-161634 (pages 7 to 8 and FIGS. 3A to 3E), Japanese Patent Laid-Open No. 10-335672 (pages 4 to 7 and FIGS. 1A to 2D), and Japanese Patent Laid-Open No. 2002-324808 (pages 7 to 10 and FIGS. 1A to 2D).
In Japanese Patent No. 3107941, it is disclosed that solution including a metal element (nickel) for promoting crystallinity of semiconductor is applied to a surface of an amorphous semiconductor film, a crystalline semiconductor film is formed by heating, and etching with solution including hydrochloric acid is performed to nickel silicide to remove nickel (nickel silicide) in the semiconductor film.
In Japanese Patent Laid-Open No. Hei 07-161634, it is disclosed that solution including a metal element for promoting crystallinity of semiconductor is applied to a surface of an amorphous semiconductor film, a crystalline semiconductor film is formed by heating, pulse oscillation excimer laser is irradiated to promote crystallization, and etching into any shape is performed to the semiconductor film.
In Japanese Patent Laid-Open No. Hei 10-335672, it is disclosed that source and drain regions of a thin film transistor are doped with phosphorus and heating at a temperature from 450 to 700° C. is conducted to remove a metal element for promoting crystallization of the semiconductor film from a channel forming region of a thin film transistor.
In Japanese Patent Laid-Open No. 2002-324808, it is disclosed that a semiconductor film (hereinafter, a semiconductor film B in the present paragraph) including a rare gas element such as argon at a concentration from 1×1019 to 1×1022/cm3 is formed over a crystalline semiconductor film (hereinafter, a semiconductor film A in the present paragraph) with a barrier film (a film such as a silicon oxide film) provided therebetween, a metal element for promoting crystallization is moved from the semiconductor A to the semiconductor B, and the semiconductor film B is removed to remove the metal element from the crystalline semiconductor film (the semiconductor film A) of a thin film transistor.
In the gettering process to the metal element, disclosed in Japanese Patent No. 3107941, however, the process (such as laser irradiation) for planarization of asperity of the surface is needed additionally since only the metal compound in the semiconductor film, typically, nickel silicide, is removed. Further, only nickel silicide separating out on the surface of the semiconductor film can be removed, which has trouble that no nickel silicide in the semiconductor film can be removed.
In Japanese Patent Laid-Open No. Hei 07-161634, pulse oscillation laser such as excimer laser is used in irradiation for promoting crystallization after heating the amorphous semiconductor film. Since laser light emitted from pulse oscillation laser has a low energy density, the semiconductor film has a region melted by irradiation of laser light, specifically, an amorphous portion and a surface of the semiconductor film. Consequently, a crystalline region of the semiconductor film is not melted, which has trouble of a metal compound, typically, nickel silicide, remaining in the crystalline region.
The gettering processes disclosed in Japanese Patent Laid-Open No. 10-335672 and Japanese Patent Laid-Open No. 2002-324808 have many processes, which has trouble of a cause of lower yield.